It is known to use a thermoplastic foam core to manufacture a variety of different sandwich panel structures. The foam core is a sheet sandwiched between outer plies, for example plies of fibre reinforced composite material. Such foam cores may be used to manufacture wind turbine blades. The core is required to have high mechanical properties, such as shear strength, compressive strength, shear modulus and compressive modulus. The foam is also required to have the correct thickness and length dimensions to form the core, sometimes the core being formed by plural interlocking foam parts extending in series along the lengthwise direction of the blade.
There is a general need to reduce the cost of the foam core. It is known to produce a foam core, for example of polyethylene terephthalate (PET) foam, by a direct extrusion process. That process has a low cost. Current PET direct extrusion foam cores have high and variable cell sizes and lower specific mechanical properties than other commercially available structural foams, such as Corecell available in commerce from Gurit (UK) Limited, which have been produced by other than an extrusion process, for example by expansion of foam pellets in a closed mould. Wind blade manufacturers use the direct extrusion PET foam for application where lower mechanical properties can be tolerated and where lower cost is a primary consideration.
The general known process to manufacture thermoplastic foam consists in extruding a continuous foam board and cutting to length at the end of the extrusion line after the board has cooled down. The foam board dimensions may vary from one manufacturer to another. Generally the thickness of the board cannot be too high because of limitations on the dimensions of the extrusion die, and because of the need to keep the board shape constant. The foam board could collapse under its own weight if the thickness is too high at the die exit.
To achieve the final foam sheet dimensions, most manufacturers are required to weld a number of foam boards together to form a block, and then the block is sliced to form a sheet, with the cell direction of the extruded foam being in the desired orientation and determined by the cutting direction. Before welding, each face of the board to be welded is required to be surfaced to ensure a high level of planarity and thus a good adhesion to the adjacent face welded thereto. The surfacing process generates over 25% of material removed (scrap for regrind) from the board. This is wasteful.
US2008/105991A1 discloses a foam manufacturing process in which, similarly, a plurality of foam boards are welded together to form a laminated block after the plurality of foam boards have been produced.
Instead of welding, it is possible to use an additional adhesion step using an applied adhesive layer to adhere adjacent foam faces together.
The use of welding or adhesion steps of the plural extruded foam boards increases the cost and complexity of the production of multi-layer structural foam laminates.
It is known to produce laminates of foamed material but the manufacturing apparatus may be complicated and expensive, and may require co-extrusion, and/or the laminates so formed may not be multi-layer structural foam laminates which are suitable, for example, to produce a laminated body of structural thermoplastic foam for use as a core material in a sandwich panel, for example in a wind turbine blade.
GB-A-1439438 discloses co-extruding laminates of foamed polymeric material from a die having plural input streams, for example in which an annular bi-layer is co-extruded around a mandrel.
GB-A-2044672 discloses a laminating process and apparatus in which preformed thermoplastic sheets are passed through a furnace in which the sheets are expanded and welded together.
U.S. Pat. No. 3,645,837 discloses the production of co-extruded thermoplastic laminates from a die having plural input streams.
JP-A-1136717 discloses the lamination of three foamed layers in the same die into a three-layered sheet.
JP-A-2008073939 discloses a foamed laminated sheet having different open cell ratios in different layers.
JP-A-2007320264 discloses a co-extruded multilayer foamed sheet.
JP-A-2000334896 discloses a multilayer foamed sheet.
JP-A-11320738 discloses the production of a foamed laminated sheet in which a first resin layer is extruded from a first extruder in a sheet-like state, and then foamed by electron beam irradiation and then heating to form a first foam layer, a second resin layer is extruded from a second extruder in a sheet-like state onto the first foam layer, and then crosslinked and foamed to form a second foam layer, and further foam layers are sequentially molded, by respective further extruders, in a multilayer to the upper surfaces of the further formed layers.